This is a nationalization of PCT/KR01/00763 filed May 10, 2001 and published in English.
1. Field of the Invention
The present invention relates to a megnetorheological fluid and a process for preparing the same, more specifically, to a megnetorheological fluid in which magnetic particles are dispersed with water in oil emulsion, and a process for preparing the same.
2. Description of the Prior Art
A magnetorheological fluid, which is also called Bingham magnetic fluid, is one of intelligent materials that can reversibly control viscosity depending on the change of a magnetic field. The magnetorheological fluid is consisted of a mobile phase comprising ferromagnetic and paramagnetic particles with diameters larger than 0.1 xcexcm and oil/water emulsion. Upon the application of a magnetic field from outside, the particles are arranged by the polarization of the inside and surface of the particles to form a fibril structure. The fibril structure plays a role to increase the viscosity and to prevent the flow of the fluid, where the yield stress increases as the strength of magnetic field increases, and the fluid comes to flow when the applied shear stress is greater than the yield stress of the fluid. The responding rate of a magnetorheological fluid to a magnetic field is as fast as 10xe2x88x923 sec and reversible, which makes possible the practical application of the magnetorheological fluid in clutches, engine mounts, vibration control units, earthquake-proof equipments of the multi-storage buildings, and robotic systems.
The magnetorheological fluid is distinguished from a colloidal magnetic fluid or ferro fluid. Compared that the size of the magnetic particles of the magnetorheological fluid is generally about several to several tens of micometers, the colloidal magnetic fluid(ferro fluid) is generally known to have the particle size of 5 to 10 nm, and do not show yield stress when a magnetic field is applied. The main application area of the ferro fluid is limited to sealing and magnetic resonance systems.
To effectively apply the magnetorheological fluid to dampers and brakes of cars and trucks, the magnetorheological fluid should have high yield stress, which may be achieved by increasing the volume ratio of the magnetic particles or imposing strong magnetic fields. However, these methods are proven to be less satisfactory in the sense that the weight of the equipment and drive electricity consumption are increased when the volume ratio of the magnetic particles are increased, and the viscosity without magnetic field increases when the strong magnetic field is applied.
In this regard, several approaches have been made to develope magnetorheological fluids overcoming the defects described above and to realize their universal applications in the industry: for example, U.S. Pat. No. 2,667,237 discloses a magnetorheological fluid, in which ferromagnetic or paramagnetic particles are dispersed with a grease mobile phase of liquid, coolant, anti-oxidative gas or semi-solid state; U.S. Pat. No. 2,575,360 describes a torque transformation equipment that can be applied to clutches and brakes, together with a magnetorheological fluid in which magnetic particles(carbonyl irons) are dispersed in 50% volume fraction with a light lubricant oil that can be used in the equipment; U.S. Pat. No. 2,886,151 describes a power transferring equipment employing a fluid thin film that responds to an electric field or a magnetic field, together with a mixture of iron oxide and a lubricant grade oil with the viscosity of 2 to 20 cp as a fluid responding to the magnetic field; U.S. Pat. Nos. 2,670,749 and 3,010,471 describe the structure of a valve controlling the flow of a magnetorheological fluid including ferro, ferrite, and diamagnetic particles, where magnetic particles are dispersed with light weight hydrocarbon oils.
The magnetorheological activity of magnetorheological fluids is largely affected by the precipitation caused by gravity. One of the major causes of the precipitation is the decrease of the magnetorheological fluid stability caused by the density difference between the magnetic particle (7.86 g/cm3) and the mobile phase (silicon oil=0.95 g/cm3). The efforts to overcome this problem have been made, for example, U.S. Pat. No. 5,043,070 teaches the stabilization of the magnetorheological fluid by employing the magnetic particles coated with two layers of surfactants, which are proven to be unsatisfactory in light of effectiveness, and U.S. Pat. No. 5,64,752 teaches the minimization of the precipitation of magnetic particles by inducing a thixotropic network for the formation of hydrogen bonds by adding a thixotropic dopes into the magnetorheological fluid, which is failed in distinctive increase of the stability.
Therefore, there are strong reasons for developing and exploring a magnetorheological fluid with improved stability.
The present inventors have made an effort to provide a magnetorheological fluid with improved stability, and discovered that a magnetorheological fluid with improved stability against precipitation can be prepared by employing a mobile phase of water in oil emulsion and magnetic particles coated with hydrophilic surfactants.
A primary object of the present invention is, therefore, to provide a magnetorheological fluid including magnetic particles coated with a hydrophilic surfactant.
The other object of the invention is to provide a process for preparing the same.